Polymer-clay nanocomposites (PCNs) have received intense research
interest over the past decade, principally due to the significant
macroscopic material property enhancements (e.g., increased tensile
strength and modulus and improved permeation resistance, flame
retardation, and heat distortion characteristics) achieved at a fraction
of the loadings required for traditional fillers [1-4]. The most widely
used nanoclay is derived from [Na.sup.+]-montmorillonite (MMT), whose
2:1 layered silicate structure consists of several stacked layers
separated by a charged intergallery. Each silicate layer is 0.98-nm
thick with a lateral dimension of 400-1000 nm [5]. The sum of the
intergallery spacing and an individual silicate layer thickness is
referred to as the basal (or [d.sub.001]) spacing, which is ~11.7
[Angstrom] for natural MMT [3]. Importantly, improvements in mechanical
properties are often ascribed to the large surface area (ca. 750
[m.sup.2]/g) of MMT, which improves coupling between filler and polymer
matrix and thereby stress transfer [6]. To promote compatibility between
hydrophobic polymers (e.g., polypropylene (PP), polyethylene) and MMT,
cation exchange reactions with alkylammonium surfactants are frequently
performed to render the clay organophilic. Additionally, highly nonpolar polymers such as PP have an unfavorable enthalpic interaction with the
organoclay that prohibits diffusion of polymer into the intergallery;
consequently, a low molecular weight copolymer of polypropylene and
maleic anhydride (PP-g-MA) is often added as a compatibilizer to
facilitate intercalation and exfoliation of the organoclay and maximize
its interfacial contact with the matrix polymer [7, 8].

The dispersion of organoclay in a polymer matrix is classified
according to the degree to which individual silicate layers are
separated from one another. In the order of decreasing layer-layer
coherence, these states are referred to as agglomerated, intercalated,
or exfoliated. In an agglomerated composite, the organoclay remains
virtually unchanged from its native state. Alternatively, in an
intercalated nanocomposite, the distance between silicate layers
increases and the number of silicate layers per organoclay domain (or
tactoid) decreases, leading eventually to the idealized exfoliated
nanocomposite. Since an ideally exfoliated sample possesses the largest
surface area to volume ratio for the filler, exfoliation leads to better
phase homogeneity and the greatest improvements in thermal and
mechanical properties [9, 10]. However, complete exfoliation is
practically unrealizable, and the best dispersed systems typically
consist of a mixture of intercalated and exfoliated structures. The
ability to promote organoclay exfoliation is intimately connected to the
preparation and blending strategy of PCNs. While in situ polymerization and solution blending methods of producing nanocomposites have been
successfully employed for a number of polymers, these methods require
large quantities of expensive, environmentally unfriendly solvents. As a
result, melt-blending with high shear remains the most practical method
of preparing PCNs [4].

Although melt-blending via extrusion is by far the most common and
industrially practical blending strategy for PCNs, few research efforts
have undertaken the considerable task of optimizing the extrusion
process. Most scientific literature has focused on surface modification
of MMT [11-16] and compatibilizer type and concentration [17-21] to
maximize compatibility between the organoclay and polymer matrix. By
contrast, investigations into the role of processing history and screw
configuration are relatively scarce. In studying polyamide six
nanocomposites, Dennis et al. concluded that increasing the mean
residence time and shear intensity generally improved delamination and
dispersion, although they noted that increasing the shear intensity
beyond an optimal level led to poorer exfoliation and dispersion [22].
Wang et al. examined the influence of PP-g-MA compatibilizer and shear
stress on dispersion of a PP-clay nanocomposite in a dynamic packing
injection molding system, confirming the dual impact of chemistry and
shear in determining dispersion [23]. Wang et al. also studied the role
of the weight fraction and maleic anhydride (MA) content of the PP-g-MA
compatibilizer [17, 18] and compatibilizer molecular weight [24]. They
observed that compatibilizer to organoclay weight ratios beyond 3:1 gave
rise to similar dispersions and that the balance between the stress and
diffusivity must be considered in tandem in selecting compatibilizer
molecular weight. Furthermore, the authors found that better dispersion
was obtained by side-feeding pure PP into a blend of PP-g-MA and clay
fed directly from the feed hopper rather than loading all three
components simultaneously. They explained that such an approach afforded
additional time for PP-g-MA to penetrate the clay galleries and
exfoliate silicate layers prior to introducing the base resin. Similar
conclusions have been drawn from studies of nylon 6 and PP, which
employed a masterbatch approach to melt-blending organo clay with
compatibilizer followed by a second dilution step with the matrix
polymer [25, 26]. Other researchers have examined the effect of various
processing variables, including feed rate, temperature, and screw speed
(shear) on dispersion of organoclay in PP [27, 28]. Applying a range of
extruder conditions, Lertwimolnun and Vergnes inferred from rheology
that dispersion and exfoliation improved with increasing shear stress
and mixing time but reduced barrel temperatures [27]. Similar
conclusions were reached by Modesti et al., who claimed that the
magnitude of the shear exerted on the polymer rather than its duration
more strongly influenced exfoliation [29]. These results motivate a high
shear, low temperature extrusion process.

One of the significant challenges associated with optimizing the
extrusion conditions for preparing PCNs is the lack of common, accepted
practice for characterizing exfoliation and dispersion. For example,
X-ray diffraction (XRD), which is popular for its simplicity and
availability, is widely used to quantify the mean basal spacing between
silicate layers after processing. However, XRD is sensitive to the
orientation distribution of organoclay domains relative to the incident
X-ray beam, which affects the number of diffraction events during an XRD
scan. As a result, XRD cannot be reliably used to quantify the number of
silicate layers per tactoid, an essential parameter in characterizing
the degree of exfoliation, and it provides little information on the
state of the dispersion. Transmission electron microscopy (TEM) is often
employed as a complementary method, since it offers a direct image of
organoclay tactoids and their distribution in the polymer matrix. On the
negative side, TEM requires rigorous, time consuming sample preparation,
is limited to small sample volumes that may not represent the average
state of the sample, and offers only a qualitative view of dispersion
[30]. Rheology has proven to be a reliable means by which organoclay
dispersion may be differentiated in PCNs, as it is well-known that both
steady shear and small-amplitude oscillatory shear (SAOS) rheology are
sensitive to the microstructure of filled polymer melts [30-43]. For
example, divergent viscosity at low shear rates in steady shear rheology
and flattening of the storage modulus in the terminal regime of SAOS are
both associated with the increased solid-like response resulting from
filler-filler interactions. In the case of PCNs, those interactions
become stronger with superior exfoliation and dispersion of organoclay,
and the resulting rheological signatures more pronounced. Consequently,
rheology offers a powerful and convenient characterization tool to
systematically evaluate extruder processing conditions and their
relationship to organoclay dispersion.

Thus, we rely primarily upon rheology to appraise the dispersion of
PP-clay nanocomposites melt-blended in a twin-screw extruder.
Compatibilized (3:1 weight ratio of PP-g-MA to organoclay) and
uncompatibilized blends of 1, 3, and 5 wt% PP-clay nanocomposites were
prepared in a two-step extrusion process. For the compatibilized blends,
a masterbatch of PP-g-MA and organoclay was melt-blended in the first
step, followed by dilution with matrix PP in the second extrusion step.
Uncompatibilized blends were simply processed twice with the appropriate
ratio of organoclay and matrix PP. In both cases, the first and second
extrusion steps were each conducted using corotating or counterrotating
screws. The compatibilized masterbatch samples are contrasted with a
compatibilized sample subjected to only a single pass. Steady shear and
SAOS rheology were performed on all blends, and a novel holistic
examination of key rheological features demonstrates the power of
rheology in characterizing dispersion in PCNs. The importance of
masterbatch processing, screw rotation mode, and sequencing is discussed
within the context of these results.

Melt-blending was conducted with a Leistritz Micro 27 twin-screw
extruder (L/D = 52, D = 27 mm) to prepare 1, 3, and 5 wt% clay
nanocomposites. For all compatibilized samples, the ratio of PP-g-MA to
C15A was fixed at a weight ratio of 3:1. C15A was introduced via a
side-stuffer positioned at the halfway down the length of the screw
(26D) and set at a metering rate appropriate to the desired loading.

Most samples were prepared using a two-step extrusion process. For
the compatibilized samples, the first step blended PP-g-MA with C15A,
creating a masterbatch (MB). Compatibilized MBs were prepared in both
co- and counterrotation screw modes. In both cases, PP-g-MA was metered
at 5 lb/hr to the screws while C15A was simultaneously added via the
side-stuffer to obtain the desired ratio (75 wt% PP-g-MA and 25 wt%
C15A). A third uncompatibilized MB sample was prepared by melt-blending
in corotating mode with PP in place of PP-g-MA. Since compatibilizer is
widely known to be critical to exfoliation and dispersion, a
comprehensive examination of screw modes on uncompatibilized samples was
judged unnecessary. The barrel temperatures and screw speeds were set at
200-210[degrees]C and 200 rpm, respectively for the MB extrusion step.

In the second dilution step, each of the three MB samples were dry
mixed with appropriate amounts of PP (yielding 1, 3, and 5 wt% PCNs) and
subsequently metered to the screws at 20 lb/hr. The dilution step was
also conducted using both co- and counterrotating screws, but the barrel
temperatures and screw speeds were set to 190[degrees]C and 300 rpm,
respectively. The rationale behind the two sets of conditions was that
preparing the MBs at a higher temperature (210[degrees]C) and lower
screw rotation speed (200 rpm) would enhance diffusion of compatibilizer
into clay intergalleries, while diluting with matrix PP at a lower
temperature (190[degrees]C) but higher screw rotation speed (300 rpm)
would increase stress and promote exfoliation and dispersion. In the
subsequent discussion, samples prepared with PP-g-MA in the two-step
process will be notated according to the screw rotation mode employed in
each extrusion step. For instance, a compatibilized sample prepared with
a counterrotating MB step and a corotating dilution step will be
referred to as "Counter-Co". Uncompatibilized samples will be
similarly described but will have the moniker "(None)" added
to specify the absence of PP-g-MA.

Finally, single-throughput 1, 3, and 5 wt% compatibilized samples,
subsequently denoted as "Co (No MB)", were also prepared in
corotating mode by dry mixing PP and PP-g-MA and metering it to the
screws at a rate of 5 lb/hr. C15A was again introduced via the side
stuffer. Matrix PP and a blend of PP and 9 wt% PP-g-MA were also
extruded once in corotating mode and serve as control materials. These
single-throughput samples were processed at the same conditions as the
MB samples described previously. All 23 samples (as well as the three
intermediate MBs) were extruded into a water bath, pelletized, and oven
dried at 80[degrees]C for 12 hr prior to the second melt-blending step
or characterization.

The screw element stacks for the counter- and corotating twin screw
extrusions employed in this work are of paramount importance in
affecting exfoliation and dispersion of organoclay. The mixing and
dispersion activity of the corotating screw configuration mainly results
from four banks of kneading block elements with different stagger angles
(30[degrees], 60[degrees], and 90[degrees]) between successive
forward-conveying paddles and one dispersive mixing element trio. The
kneading block elements in the corotating mode are forward conveying
elements, leading to a mean residence time of 60 s. In the
counterrotating configuration, dispersive mixing arises mainly from the
four banks of helical lobe dispersive mixing element pairs and one
combing distributive mixer element trio. These groupings induce a
partial reverse conveying character that increases the mean residence
time to 105 s. Detailed schematics of the screw element stacks are
available from the manufacturer [44, 45]. For the single-throughput
compounding process and MB step, only the 28D to 52D mixing elements
(i.e., those downstream of the side-stuffer where organoclay is first
introduced) contribute to the dispersion and exfoliation of nanoclay
particles. For the second step of the two-step processes, all mixing
elements contribute to clay exfoliation and dispersion.

X-Ray Diffraction

Samples used in XRD experiments were first compression molded at
210[degrees]C to a thickness of ~1.5 mm. XRD experiments were conducted
using a Scintag XDS 2000 diffractometer with a Cu K[alpha] radiation
source ([lambda] = 1.540562 [Angstrom]). Data were collected with a step
size of 0.02[degrees] and a scan rate of 0.5[degrees]/min. The results
presented are the average of two scans. The angle at which the primary
peak in diffraction intensity is observed corresponds to the mean basal
spacing of the organoclay, which is computed from Bragg's Law.

Rheology

All rheological tests were conducted with a TA Instruments AR 2000
rheometer using a 25-mm parallel plate geometry and environmental test
chamber (ETC). A nitrogen purge of 10 l/min was continuously supplied to
the ETC to inhibit oxidative degradation of the PP during experiments.
Compression molded samples were melted at 210[degrees]C for 10 min in
the rheometer, and the upper plate was then lowered to a gap distance of
1 mm for testing. All experiments were performed at 180[degrees]C.

Each sample was subjected to three types of rheological
experiments: (1) oscillatory strain sweeps, (2) SAOS frequency sweeps,
and (3) steady shear rate sweeps. These tests were selected because of
their sensitivity to filler loading and dispersion and the unique
experimental signatures associated with the development of a solid-like
mechanical response. First, oscillatory strain sweeps were performed at
a fixed frequency of 1 rad/s by varying the amplitude of the applied
strain ([[gamma].sub.0]) from 0.001 to 10, although it should be noted
that reliable data were only obtained up through a strain of ~8, beyond
which melt fracture at the edges of the parallel plate geometry occurs.
Typically, oscillatory strain sweeps serve to identify the critical
strain corresponding to the boundary between linear and nonlinear viscoelastic behavior. For PCNs, several authors have reported that the
critical strain ([[gamma].sub.c]) is inversely proportional to the
volume fraction of clay, though the steepness of the dependence is
influenced by the degree of exfoliation and dispersion [39, 41, 46].
Here, we define the critical strain as the strain at which the complex
viscosity has decreased to 90% of its linear viscoelastic plateau value.
Additionally, the magnitude of the complex viscosity (|[eta]*|) at low
strain is sensitive to organoclay loading and dispersion. Therefore, we
will focus upon these two quantities in examining oscillatory strain
sweep data.

Numerous studies of filled polymeric liquids have cited a
flattening, or plateau, of the storage modulus (G') in the terminal
regime during SAOS frequency sweeps. Such behavior has been attributed
to the formation of a mesoscale clay network, which is sensitive to
organoclay loading, extent of exfoliation, and dispersion [37, 42, 43,
47, 48]. Moreover, while (G') tends to increase at low frequencies
as organoclay loading increases and dispersion improves, the loss
modulus (G") remains largely unaffected. As a result, the loss
tangent (tan [delta] [equivalent to] G"/G'), which is another
measure of solid-like character in a fluid, decreases in the terminal
regime. By contrast, the loss tangent of polymeric liquids grows
indefinitely with decreasing frequency. As in the case of oscillatory
strain sweeps, the complex viscosity in the terminal regime is a third
measurable by which the solid-like nature of the samples may be probed,
as it also increases with loading and dispersion. For the SAOS
experiments reported here, the strain value was chosen to be 0.01 to
ensure a linear response, and a single frequency sweep was conducted at
180[degrees]C from high to low frequency (100-0.01 Hz). While frequency
sweeps were conducted at other temperatures and time-temperature
superposition (TTS) validated, the TTS curve is not germane to this
study, and we report only the results of the sweep performed at
180[degrees]C.

Finally, steady shear rate sweeps were performed from low to high
shear rate (0.01-10.0 [s.sup.-1]) at 180[degrees]C. Again, we note that
some high shear rate data were discarded as unphysical, probably
resulting from edge fracture in the sample. Prior work has shown that
organoclay loading and dispersion can cause PCNs to behave as constant
shear-thinning materials [37, 39, 43, 49]. That is, they exhibit a
monotonically decreasing viscosity even at low shear rates where the
matrix polymer would otherwise plateau to the zero-shear viscosity. This
divergence of viscosity at low shear rates (defined so because the
viscosity asymptotes to infinity in the limit of zero shear rate) is
consistent with the presence of a finite yield stress and interpreted as
a strong indicator of the hypothesized volume-spanning, mesoscale clay
network [37, 38, 47, 49]. The yield stress may be inferred by fitting
the low shear rate data to Casson's equation, given by:

where [[sigma].sub.xy] is the shear stress, [[sigma].sub.0] is the
yield stress, and the parameter a is an arbitrary constant [39, 43, 50].
Upon imposition of shear, the highly anisotropic organoclay domains tend
to orient with the flow direction even at low shear rates, because the
characteristic time scale for rotational diffusivity is typically much
longer than that associated with the deformation (i.e., the reciprocal
of the shear rate). Flow-induced orientation of organoclay domains leads
to concurrent disruption of the network and yield in the sample.
Moreover, when organoclay domains increasingly orient in the flow
direction, their projection of orientation in the velocity gradient
direction diminishes, resulting in a monotonically decreasing viscosity.
Thus, for highly filled, well-dispersed samples, flow-induced
orientation of organoclay domains and divergent shear thinning behavior
are expected. On the other hand, samples with low clay loadings and poor
exfoliation are expected to exhibit the liquid-like rheology
characteristic of the polymer melt with only a modest increase in the
zero-shear viscosity resulting from the inclusion of a dilute
concentration of solid particles. In summary, the yield stress and
steady shear viscosity at low shear rates offer two more experimental
markers by which organoclay dispersion may be probed rheologically.

RESULTS

X-Ray Diffraction

The results of XRD scans for all 1, 3, and 5 wt% PCNs are shown in
Fig. 1. It is immediately apparent that the non-MB and uncompatibilized
samples are characterized by stronger diffraction peaks at all
organoclay loadings. While this result does not certify that these
materials are less well exfoliated than the others, the increase in the
number of diffraction events at a particular angle indicates a greater
coherence between silicate layers with the requisite orientation
relative to the X-ray beam. Hence, the XRD scans are in accord with
expectations that MB processing with compatibilizer should enhance
exfoliation in processing.

Table 1 summarizes the organoclay basal spacing values calculated
from the angle associated with the peak intensities in Fig. 1. We first
note that the uncompatibilized samples possess basal spacings markedly
smaller than all compatibilized samples, as expected, as well as the
as-received C15A organoclay (3.222 nm). The reduction relative to C15A
may result from degradation of surfactant molecules, which is known to
occur at temperatures above 200[degrees]C [51, 52]. Since PP is
thermodynamically prohibited from diffusing into the intergallery,
thermal degradation of surfactant causes a decrease in basal spacing.
Additionally, within the uncompatibilized samples, the [d.sub.001]
spacing of the sample processed in counterrotation mode for the dilution
step is consistently smaller than that processed with corotating screws
during dilution. Since the dilution step is the only difference in
preparation between these two samples, the shift to smaller basal
spacing for the Co-Counter (None) sample highlights the importance of
processing history even in the absence of compatibilizer. Whether the
difference in basal spacing translates to changes in dispersion and
solid-like rheology will be discussed in the subsequent sections.

The [d.sub.001] spacings of the compatibilized samples are similar
to that of the as-received C15A clay. While some are slightly larger,
others are somewhat smaller with no definitive dependence upon
processing history and organoclay loading. Furthermore, the basal
spacings of the single-throughput samples are indistinguishable from
those of the compatibilized MB blends. Hence, we do not believe that the
results for the compatibilized samples are indicative of any meaningful
differences in organoclay structure, at least on the length scale probed
by XRD. Because the organically modified C15A already has a large
[d.sub.001] spacing (relative to the 1.17 nm of natural MMT), additional
swelling of the intergallery without exfoliating layers may be difficult
to achieve. It should be noted that surfactant degradation will also
occur in the compatibilized samples, and differences in basal spacing
between compatibilized and uncompatibilized blends highlight the ability
of the PP-g-MA compatibilizer to expand the clay intergallery. However,
because XRD cannot generally provide information about the number of
platelets per tactoid, it is a poor standalone technique for assessing
the degree of exfoliation [43, 53]. Consequently, changes in organoclay
structure at the mesoscale, which may be inferred from rheology, are
more likely to reveal the effect of processing history on exfoliation
and dispersion. Therefore, we simply note that the intergallery
distances of the compatibilized samples remain larger than those of the
uncompatibilized counterparts, suggesting that PP-g-MA successfully
intercalated between silicate layers.

[FIGURE 1 OMITTED]

Rheology

As stated previously, rheology has proven to be an effective means
of indirectly probing clay exfoliation and microstructure [9, 35-40, 49,
54]. In this section, we will present data and identify qualitative
features and trends upon which we will elaborate in the Discussion
section to follow. Frequently, we will refer to samples as having more
or less solid-like rheology. This designation implies greater
microscopic interaction between organoclay domains, which leads to
increases in viscosity and storage modulus and decreases in loss
tangent. For a given organoclay loading, the increased interactions
between organoclay domains that contribute to these rheological
signatures can only derive from improved exfoliation and dispersion.
Although a detailed analysis of the forthcoming results in the context
of exfoliation and dispersion will be deferred to the Discussion
section, it is helpful to retain a vision of the underlying source of
the rheological features identified below.

We begin with oscillatory strain data for all three organoclay
loadings presented in Fig. 2, where we draw attention to the magnitude
of the complex viscosity in the linear viscoelastic (low strain
amplitude) regime and the critical strain at which the transition to
nonlinear rheology occurs. First, in accordance with expectations, we
observe that the dependence of the low strain complex viscosity on
processing history becomes stronger with organoclay loading. However,
more pertinent to the goals of this study are the trends that develop
with regard to processing history and sample formulation. At 1 wt%, most
PCN samples are not noticeably distinct from the matrix PP, and most
observed rheological differences are likely due to changes in polymer
composition rather than organoclay exfoliation and dispersion. For
instance, the uncompatibilized materials exhibit a higher low strain
complex viscosity than all but the compatibilized Counter-Co sample.
Since the low strain complex viscosity of the PP/9 wt% PP-g-MA control
sample is also less than that of the matrix PP, it is clear that the low
molecular weight compatibilizer is more strongly affecting the
viscoelastic response than the organoclay. As a result, the low strain
complex viscosity of the compatibilized samples tends to be less than
that of the uncompatibilized blends. The lone exception is the
Counter-Co MB sample, which has the highest low strain complex viscosity
and smallest critical strain for shear thinning, pointing to a stronger
organoclay influence.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

For the 3 wt% samples shown in Fig. 2b, the state of the organoclay
dispersion plays a prominent role. The four compatibilized MB samples
possess low strain complex viscosities above those of the two
uncompatibilized samples and appear to shear thin at smaller strains as
well. Of the four compatibilized MB samples, Counter-Co again possesses
the highest low strain complex viscosity. It is also interesting to note
a mixed influence of organoclay on the results for the single-throughput
Co (No MB) sample. Despite containing PP-g-MA, its low strain complex
viscosity is only slightly less than that of Co-Co (None), suggesting
fewer clay-clay interactions and a poorer dispersion, yet its critical
strain appears to be smaller indicating the converse. The trends of the
3 wt% samples continue at the 5 wt% organoclay loading. All five
compatibilized materials have substantially higher low strain complex
viscosities and lower critical strains than the uncompatibilized
samples. In this case, Co-Co supplants Counter-Co as the sample with the
highest low strain complex viscosity, although it is important to note
that all four MB materials exceed the single-throughput compatibilized
sample by a wide margin in this regard, emphasizing the efficacy of the
MB processing approach.

[FIGURE 4 OMITTED]

In SAOS, a transition from liquid-like (i.e., G'
[proportional] [[omega].sup.2] and G" [proportional] [omega]) to
solid-like (i.e., G', G" [proportional] [[omega].sup.0])
behavior in the terminal regime has been related to the formation of a
solid-like organoclay network [36, 37, 40-43, 47]. In Figs. 3-5, we
present data for the storage modulus, loss tangent, and complex
viscosity for the 1, 3, and 5 wt% PCNs, respectively. The data at 1 wt%
organoclay loading are similar to those for oscillatory strain in the
sense that the rheological response of most samples is unaffected by
organoclay, displaying a liquid-like, polymeric response that includes
the expected slope of 2 for the storage modulus, diverging loss tangent,
and plateau in complex viscosity in the terminal regime. Again, the
Counter-Co sample is the exception to this description. Its shallower
G' slope, flattening of the loss tangent, and modestly diverging
complex viscosity are all signatures of solid-like rheology.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Shifting to the 3 wt% samples in Fig. 4, the qualitative
differences between the compatibilized MB, single-throughput
compatibilized, and uncompatibilized samples are manifest. All
compatibilized MB materials exhibit significant flattening of G',
decrease in the loss tangent, and increased and diverging complex
viscosity in the terminal regime relative to uncompatibilized samples.
Moreover, the effects are accentuated for the MB samples relative to the
single-throughput sample. Differentiating between the MB samples is more
challenging, although the results for storage modulus and complex
viscosity suggest that the Co-Counter sample is less solid-like than the
others. Interestingly, Co-Counter (None) is also less solid-like than
Co-Co (None). Finally, similar trends prevail for the 5 wt% blends in
Fig. 5. The four compatibilized MB samples, which are almost
indistinguishable from one another graphically, are unequivocally more
solid-like than the uncompatibilized samples, whereas the
single-throughput sample falls in between. Also, the differences in
rheology between Co-Counter (None) and Co-Co (None) are even more
pronounced, where the latter exhibits much more solid-like behavior in
the terminal regime. We will return to this point when discussing the
impact of processing sequence.

In Fig. 6, we present the final set of rheological data for steady
shear rate sweep experiments of 1, 3, and 5 wt% PCNs. Similar to the
SAOS complex viscosity data, we focus upon increases in viscosity at low
shear rates as well as the appearance of divergent behavior, both of
which are believed to derive from a mesoscale organoclay network [2, 37,
39, 49]. The results for 1 wt% organoclay mirror the earlier
observations for complex viscosity in oscillatory strain and SAOS at the
same loading. Compatibilizer seems to have a stronger effect than
organoclay on the viscosity, except for the Counter-Co material, which
again exhibits a higher viscosity. At 3 wt%, only the uncompatibilized
samples show the low shear plateau characteristic of unfilled polymeric
fluids, although the fact that the zero-shear viscosity of the Co-Co
(None) sample exceeds that of Co-Counter (None) again points to a strong
processing history influence in the uncompatibilized samples.
Alternatively, all compatibilized samples show the divergence in
viscosity characteristic of a finite yield stress. Since we are focusing
on qualitative features at this time, we will defer discussion of the
yield stresses inferred from Casson's equation to the Discussion
section. The viscosity of all four MB samples at any given low shear
rate exceeds that of the single-throughput compatibilized blend.

DISCUSSION

In the preceding section, a large amount of rheological data was
presented from which we may draw the following qualitative conclusions
concerning the processing of PP-clay nanocomposites:

* The addition of PP-g-MA as a compatibilizer leads to more
solid-like rheology for 3 and 5 wt% organoclay loadings, presumably due
to its ability to intercalate between silicate layers and facilitate
exfoliation and dispersion. At a 1 wt% loading, the effect of the low
molecular weight PP-g-MA on exfoliation and dispersion has a minimal
influence on rheology, but it does tend to decrease the small
deformation viscosity except in the case of the compatibilized
Counter-Co sample.

* Two-stage MB processing, in which organoclay is first
melt-blended with PP-g-MA compatibilizer before subsequent dilution with
matrix polymer, improves exfoliation and dispersion (as inferred from
rheology) relative to single-throughput processing at 3 and 5 wt%
loadings. At 1 wt%, the effect is again indeterminate, except for the
Counter-Co blend.

While important, these conclusions are not entirely unexpected. To
comment specifically upon the more subtle influences of screw rotation
mode and sequencing, a more detailed, quantitative analysis of these
data is required. However, analyzing data for three types of rheological
experiments performed on a total of 23 samples is a potentially daunting task. The challenge lies in identifying key quantities that capture the
behavior of greatest interest. We will focus on seven quantities
extracted from the three types of experiment that are believed to best
reflect the rheological changes engendered by improved exfoliation and
dispersion. For oscillatory strain sweeps, we will examine the plateau
value of the complex viscosity at small strain amplitude (0.01) and the
critical strain marking the boundary between linear and nonlinear
viscoelasticity. For SAOS, the storage modulus, loss tangent, and
complex viscosity at 0.01 Hz will be included. Lastly, the shear
viscosity at low shear rate and the yield stress inferred from
Casson's equation will be used. The shear rate at which the
viscosity is taken will depend upon the organoclay loading, because the
rheometer is able to access lower shear rates as the sample viscosity
increases. Hence, the reported values of shear viscosity for the 1, 3,
and 5 wt% samples will correspond to those recorded at shear rates of
0.0251, 0.0126, and 0.01 [s.sup.-1], respectively.

In Tables 2-4, we present the quantitative values of the seven
rheological parameters for the three organoclay loadings. Table 2 also
contains similar data for the processed PP and PP/9 wt% PP-g-MA control
samples. However, while quantitative measures of organoclay exfoliation
and dispersion are now summarized in tabular form, a strategy for
averaging the results for a given processing history and sample
composition is necessary to use rheology to holistically assess the
influence of those variables. Therefore, we have taken the results in
Tables 2-4 and normalized them to a unit scale, where zero represents
the most liquid- or polymer-like response and unity the most solid-like,
well exfoliated, and dispersed result. For example, consider oscillatory
strain sweep experiments at fixed organoclay loading, where an increase
in complex viscosity is indicative of increased clay-clay interactions
and improved exfoliation and dispersion. Hence, its normalized value is
calculated as (|[eta]*| - |[eta]*|[.sub.min])/(|[eta]*|[.sub.max] -
|[eta]*|[.sub.min]), where |[eta]*|[.sub.max] and |[eta]*|[.sub.min]
represent the maximum and minimum values of all the samples at that
organoclay loading. All quantities for which a large value of the
parameter represents improved exfoliation and dispersion are computed in
this fashion. For the critical strain and loss tangent, smaller values
reflect better exfoliation and dispersion; consequently, the normalized
critical strain, for instance, is calculated as ([[gamma].sub.c,max] -
[[gamma].sub.c])/([[gamma].sub.c,max] - [[gamma].sub.c,min]). Each
organoclay loading is normalized and evaluated independently of the
others; however, because the processed PP and PP/9 wt% PP-g-MA represent
control materials, they are included in the normalizations at each
organoclay loading.

These normalized parameters are reported in Tables 5-7 along with
the ordinal ranking for that parameter. Additionally, an average score
is reported for each blend, where equal weight is given to each type of
experiment (not each rheological parameter) to address the imbalance in
the number of parameters extracted from each experiment. The expectation
is that the average score will provide a simple and effective
quantitative assessment of the effect of both compatibilization and
processing history on organoclay exfoliation and dispersion. Beginning
with the results for the 1 wt% organoclay loading, we note quantitative
support for the observation that the compatibilized Counter-Co sample
was more solid-like than the other 1 wt% materials. At first glance, it
is surprising that the uncompatibilized samples ranked second and
fourth; however, we recall that the rheology at this organoclay loading
is dominated by polymer rather than clay-clay interactions. Hence, the
low molecular weight PP-g-MA significantly reduces the viscosity of
compatibilized blends, invalidating a holistic analysis predicated on
clay-clay interactions having greater influence on rheology. The lone
rheological signature that is not affected by PP-g-MA is the yield
stress. Interestingly, if the results were sorted based solely on the
ordinal values of yield stress, the four compatibilized MB materials
would occupy rankings one through four, followed by the
single-throughput compatibilized blend and finally the two
uncompatibilized samples. Within the compatibilized samples, Counter-Co
has the highest yield stress, followed distantly by Counter-Counter,
Co-Co, Co-Counter, and Co (No MB). Although the yield stresses at this
low loading are relatively small and the fitting to Casson's
equation somewhat subjective, we will recall this ranking based on yield
stress when considering holistic rankings at higher loadings. In
summary, while the holistic rheological analysis of the 1 wt% materials
has been shown to be inadequate when clay-clay interactions do not
dominate rheological behavior, rankings based upon yield stress produce
results consistent with expectations.

Turning next to the 3 wt% samples whose normalized values are shown
in Table 6, we see that the holistic rheological analysis is more robust
at differentiating between not only compatibilized and uncompatibilized
samples but also screw rotation mode and sequence. First, the
uncompatibilized blends are less solid-like than the compatibilized
blends based on average score and nearly all individual parameters,
reinforcing the premise that PP-g-MA facilitates exfoliation and
dispersion of clay silicates. The four MB samples similarly distinguish
themselves relative to the Co (No MB) sample. As a result, it appears
that MB processing generates better exfoliated and dispersed materials,
likely due to the prolonged direct mixing between PP-g-MA and organoclay
before PP is introduced. Examining the individual and average scores of
the four compatibilized MB blends, we observe stronger dependence on
processing history than at the 1 wt% loading. Intriguingly, however, the
rankings of their average scores match those reported above for 1 wt%
based solely on yield stress. That is, Counter-Co proves to be the most
solid-like, followed by Counter-Counter, Co-Co, and Co-Counter at
discrete intervals. Put another way, the counterrotation processing of
PP-g-MA and organoclay in the MB step leads to better exfoliation and/or
dispersion in the final product. Counterrotating screws are known to
impart significantly higher stresses over longer extruder residence
times relative to corotating screws (~75% longer in this case). It is
possible that the longer residence time in counterrotation mode allows
the compatibilizer more time to diffuse into clay intergalleries, while
the higher stresses facilitate the exfoliation of clay silicates. The
manner in which material is processed in the subsequent dilution step
also appears to be an important consideration. Here, we compare the
results for fixed MB processing conditions but varying dilution
conditions (e.g., Counter-Co vs. Counter-Counter, and Co-Co vs.
Co-Counter). In both cases, corotating screws in the dilution step yield
more solid-like materials than counterrotating screws, a result that is
also reflected in the uncompatibilized samples. The explanation for this
behavior may lie in the thermomechanical degradation of PP during
extrusion, which is known to be accelerated at higher stresses, longer
residence times, and in the presence of filler [55-58]. The result is
[beta]-scission, a reduction in the average molecular weight of the
matrix PP, and a corresponding decrease in viscosity. Since the
zero-shear viscosity of the matrix PP used in this study decreased by
nearly a factor of two after extrusion with corotating screws (data not
reported), thermomechanical degradation is important during the
processing of these blends. The stresses experienced by compatibilized
MB samples in the dilution step should be larger due to the presence of
organoclay, which increases the stress imparted by the screws by virtue
of higher sample viscosity. For identical MB processing conditions, both
corotating and counterrotating screws will induce thermomechanical
degradation, but the higher stresses of the counterrotating mode
undoubtedly exacerbate the effect. A more pronounced decrease in matrix
viscosity during the counterrotating dilution step would reduce the
shear stress transfer between the polymer melt and clay tactoids,
leading to less effective exfoliation and dispersion. As a result,
materials processed in corotating mode for the dilution step tend to
have more solid-like rheological properties. Finally, we noted a similar
pattern in the uncompatibilized Co-Co (None) and Co-Counter (None)
blends, where the former exhibited more solid-like rheology than the
latter at all loadings in all experiments. The most significant
reduction in basal spacing was also observed for the Co-Counter (None)
blend. We suspect that the combination of smaller separation between
silicate layers and greater thermomechanical degradation both contribute
to the less solid-like rheology of the Co-Counter (None) sample,
although their relative importance is likely dependent upon organoclay
loading.

Finally, we examine the normalized rheological data for the 5 wt%
blends summarized in Table 7. While the average scores of the
compatibilized MB, uncompatibilized, and single-throughput
compatibilized materials remain quite distinct from one another, we
immediately note much less differentiation between the four MB samples
than at 3 wt%. Both the average and individual parameter scores for the
MB blends are compressed into a narrow range of values, and the ordinal
rankings are inconsistent across parameters. Hence, although the
Co-Counter sample (heretofore the least solid-like of the four)
unexpectedly scores the highest and Counter-Counter the lowest of the
four, the individual parameters (see Table 4) are often not meaningfully
different from one another. For example, it is difficult to argue that
the reported values of critical strain and loss tangent are appreciably different from one another (a fact that applies to a lesser degree for
the 3 wt% samples as well), which renders parameters with greater
variation more influential in computing the average. However, the
inability of the holistic rheological analysis to resolve
microstructural differences in the four compatibilized samples at 5 wt%
organoclay loading may itself represent an important result. At high
organoclay loadings, screw rotation mode and sequence (and thereby
stress and residence time in the extruder) may not significantly impact
the degree of exfoliation and dispersion of organoclay. In this more
highly percolated state, clay-clay interactions are the dominant
rheological influence, as evidenced by the over 20-fold increase in
shear viscosity relative to the processed PP. However, subtle changes in
the degree of exfoliation may have only a secondary effect on
rheological properties, resulting in the capricious parameter rankings
in Table 7. Nonetheless, it is apparent that the compatibilized MB
processing yields a more solid-like product than the compatibilized
single-throughput and uncompatibilized blends, serving notice that
introduction of compatibilizer in the MB processing step has a profound
impact on organoclay exfoliation and dispersion.

CONCLUSIONS

PP-clay nanocomposites with 1, 3, and 5 wt% organoclay loadings
were studied with the goal of elucidating the importance of MB
processing, screw rotation mode, and sequence. XRD data demonstrate that
uncompatibilized samples have smaller basal spacings than compatibilized
blends, but the technique cannot differentiate the latter on the basis
of processing history. On the other hand, a holistic rheological
analysis of oscillatory strain, frequency, and steady shear rate sweep
experiments has been shown to be sensitive to differences in organoclay
exfoliation and dispersion caused by variation in processing history.
Results not only support the well-known premise that PP-g-MA
compatibilizer dramatically improves exfoliation and dispersion of clay
silicates but also demonstrate that MB processing of compatibilizer and
organoclay followed by dilution with matrix PP has a similarly
efficacious effect. Moreover, at the 1 and 3 wt% organoclay loadings,
the higher stress and longer residence time characteristic of the
predominantly dispersive mixing counterrotation screw mode appear to
improve exfoliation in the MB step, whereas the greater distributive
mixing of a corotating screw configuration in the dilution step may
reduce thermomechanical degradation relative to counterrotating screws.
At the higher 5 wt% loading, the sequence of processing is less
important, suggesting that the sheer volume of organoclay renders subtle
changes in the degree of exfoliation and dispersion less consequential to the rheology.

ACKNOWLEDGMENTS

We thank Dr. Robert Sammler of The Dow Chemical Company for the
generous donation of the H700-12 polypropylene resin, Dr. Leigh Allen of
Crompton Corporation for the Polybond[R] 3200 PP-g-MA compatibilizer,
and Dr. Douglas Hunter of Southern Clay Products for the Cloisite[R] 15A
montmorillonite clay.